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Wow, I don't know what I was thinking. Sorry about that! I'll explain what it is in an answer...
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AtaraxiaSep 7 '13 at 13:10

Note that the imperative form of なさる, namely なさい which you are talking about, is irregular. Its regular imperative form, なされ, is theoretically possible, but it sounds archaic or colloquial.
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PteromysSep 9 '13 at 14:53

2 Answers
2

しなさい is a verb conjugation that turns the verb する into a command (imperative form). There are a number of imperative forms, but this one in particular gives the nuance of "talking down" or giving advice that you feel is helpful to the listener (but keep in mind, it carries the connotation that you know better than the listener). This is generally how a parent would speak to his or her child, or a teacher would talk to students.

If you're not sure whether or not it's appropriate to use in a given situation, imagine saying it in English with the phrase "You had better". e.g:

勉強しなさい
(You had better) study.

[図書館]{としょかん}で[静]{しず}かに[喋]{しゃべ}りなさい。
(You had better) talk quietly in the library.